Catalytic cracking is an established and widely used process in the petroleum refining industry for converting oils and residua of relatively high boiling point to more valuable lower boiling products including gasoline and middle distillates such as kerosene, jet fuel and heating oil. The pre-eminent catalytic cracking process now in use is the fluid catalytic process (FCC) in which the pre-heated feed is brought into contact with a hot cracking catalyst which is in the form of a fine powder, typically with a particle size of 10-300 microns, usually about 100 microns, for the desired cracking reactions to take place. During the cracking, coke is deposited on the catalyst and this results in a loss of activity and selectivity. The coke is removed by continuously removing the coked, spent catalyst from the cracking reactor and oxidatively regenerating it by contacting it with air in a regenerator. The combustion of the coke is a strongly exothermic reaction which, besides removing the coke, serves to heat the catalyst to the temperatures appropriate for the endothermic cracking reaction. The process is carried out in an integrated unit comprising the cracking reactor, the regenerator and the appropriate ancillary equipment. The catalyst is continuously circulated from reactor to regenerator and back to the reactor with the circulation rate being adjusted relative to the feed rate of the oil to maintain a heat balanced operation in which the heat produced in the regenerator is sufficient for maintaining the cracking with the circulating, regenerated catalyst being used as the heat transfer medium. Typical fluid catalytic cracking processes are described in the monograph Fluid Catalytic Cracking with Zeolite Catalysts, Venuto, P. B. and Habib, E. T., Marcel Dekker Inc., N.Y. 1979, to which reference is made for a description of such processes. As described in the monograph, the catalysts which are currently used are based on zeolites, especially the large pore synthetic faujasites, zeolites X and Y, which have generally replaced the less active, less selective amorphous and clay catalysts formerly used.
Another catalytic cracking process still used in the industry is the moving, gravitating bed process, one form of which is known as Thermofor Catalytic Cracking (TCC) which operates in a similar manner to FCC but with a downwardly moving gravitating bed of a bead type catalyst, typically about 3-10 mm in diameter. Fixed bed units have now been replaced by moving or fluidized bed units of the FCC or TCC type.
The feed to the catalytic cracker can generally be characterized as a high boiling oil or residuum, either on its own or mixed with other fractions, usually of a high boiling point. The most common feeds are gas oils, that is, high boiling, non-residual oils with an initial boiling point usually above about 230.degree. C. (about 450.degree. F.), more commonly above about 345.degree. C. (about 650.degree. F.), with end points of up to about 620.degree. C. (about 1150.degree. F.). Typical gas oil feeds include straight run (atmospheric) gas oil, vacuum gas oil and coker gas oil; residual feeds include atmospheric residua, vacuum residua and residual fractions from other refining processes. Oils from synthetic sources such as Fischer-Tropsch synthesis, coal liquefaction shale oil or other synthetic processes may also yield high boiling fractions which may be catalytically cracked either on their own or in admixture with oils of petroleum origin.
The ease with which any given cracking feed is cracked and the selectivity for the desired products depends partly upon the composition of the feed. Virgin petroleum stocks which have not previously been subjected to cracking tend to crack relatively easily because they possess long chain alkyl groups which, by a process of dealkylation which occur readily during cracking, form lower boiling products. The aromatic residues which are left following dealkylation are more highly refractory so that catalytic cracking cycle oils, e.g. LCO or HCO, generally require severe hydrotreating to saturate them before they can be cracked to any significant extent under conventional conditions. Besides improving crackability, especially of these highly aromatic cycle oils, hydrotreating has been recognized as useful for other purposes, including demetallation and, above all for desulfurization and denitrogenation, both of which are desirable to improve product quality, catalyst selectivity and aging rate as well as reducing emissions, principally SO.sub.x, from the regenerator. Use of a feed hydrotreater has been reported to result in higher conversion and gasoline yield, lower coke make per pass, more favorable light gas distribution, higher isobutane yield, and lower contaminant content in products and unit off-gases: cf. the Venuto/Habib monograph and the Oil and Gas Journal, May 19, 1966, 131-139; Oct. 14, 1974, 99-110; July 21, 1975, 53-58.
In general, the hydrotreating of catalytic cracking feedstocks has been carried out at relatively low pressures, below about 7000 kPa (about 1,000 psig) and in most cases below about 5,500 kPa (about 785 psig) as the use of higher pressures does not enhance the desulfurization activity which has, for the most part, been the principal objective of cracking feed hydrotreating. Denitrogenation has generally followed with the desulfurization to some degree, depending upon the composition of the feed and the severity of the processing although more severe processing conditions are required for nitrogen removal than sulfur removal. The hydrogenation of aromatics is known to be favored by the use of higher hydrogen partial pressures although the response of aromatic compounds differs according to their composition: condensed ring aromatics may be hydrogenated at lower pressures than non-condensed, polycyclic aromatics and although the hydrogenation of both is favored by the use of relatively low temperatures (since hydrogenation is an exothermic reaction) the optimum temperature for hydrogenating each of these types varies (Oil and Gas Journal July 21, 75, 53-58). The use of lower temperatures, however, does not favor sulfur or nitrogen removal because those reactions require a certain measure of cracking (an endothermic reaction) and the same is true of demetallation although, in most cases, a relatively mild treatment will suffice for metals removal. The choice of conditions for the hydrotreating of catalytic cracking feeds has therefore represented a compromise between different competing factors without a significant attempt at optimizing the conditions in a way which permits the greatest advantages to be obtained at minimum cost. In the case of hydrotreating, minimum cost implies minimum hydrogen consumption and maximum catalyst cycle life.